Membrane receptor molecules play a central role in many cellular processes including growth control, differentiation and hormone responsiveness. Membrane immunoglobulin M (IgM) is the receptor for a developmental signal which stimulates a small B lymphocyte to differentiate into an IgM secreting plasma cell. The structure of the membrane IgM glycoprotein has recently been determined and compared with secreted IgM by using B-lymphocyte tumors from inbred mice. These two forms are identical except for their carboxy termini. The membrane form has a hydrophobic "tail" for membrane interaction, and the secretable form has a hydrophilic "tail" which allows it to form multimers and to be secreted. One gene encodes the constant region of mu; separate gene segments encode the alternative carboxy termini. It is thought that the two forms of mu are produced by alternative RNA splicing of a single primary transcript. We have used mouse tumor cells which express both membrane (mIgM) and secretable (sIgM) forms. From these cells we have immunoselected more than 50 variants which no longer express the mIgM. Six of these have been extensively characterized with respect to rate, amount, stability and glycosylation of mum and mus protein andamount and rate of mRNA encoding mum and mus. These studies have allowed us to better define the requirements for expression of mIgM and sIgM.